1. Field of the Invention
The present invention relates to a lithotrity apparatus for externally irradiating a stone in a patient's body with a focused impulse emitted from an impulse source such as a piezoelectric device to perform lithotrity treatment, and more particularly, to a lithotrity apparatus having a missed-shot preventive function which prevents an impulse from erroneously irradiating a normal tissue.
2. Description of the Related Art
In treatment of mainly a renal calculus, a method of externally irradiating a stone in a body with a focused impulse to pulverize the stone for non-invasive treatment has recently begun to be widely used. Treatment methods which use underwater discharge, electromagnetic induction, micro explosion, a piezoelectric device and the like as an impulse source are known well.
According to one that uses a piezoelectric device, among these methods, an echo sent from a focal zone of the impulse can be received by the piezoelectric device which is also used for impulse generation. A technique which utilizes this function to prevent missed-shot during treatment is conventionally proposed (for example, refer to Japanese Patent Application Nos. 60-191250 and 61-149562). According to a lithotrity apparatus which employs this technique, immediately before an intense impulse for lithotrity treatment is emitted, an ultrasonic wave weaker than that (to be referred to as a low-pressure wave hereinafter) is sent and received. When an intense echo is returned, it is determined that the focal point coincides with the stone. Then, the piezoelectric device is driven by a high voltage to irradiate a target, i.e., the stone with an intense impulse.
FIGS. 3(a) to 3(d) are timing charts for explaining an example of a missed-shot preventive method in accordance with a conventional technique. FIG. 3(a) is a timing chart of an irradiating rate pulse having an impulse irradiating interval T; FIG. 3(b) is an irradiating timing chart of a low-pressure wave; FIG. 3(c) is a timing chart of a received echo; and FIG. 3(d) is a timing chart of an impulse irradiating timing.
The echo of FIG. 3(c) is compared with a threshold Vth. The piezoelectric device is driven by a high voltage only when the echo intensity is the threshold Vth or more. Then, an impulse is emitted at a timing as shown in FIG. 3(d). When the echo intensity is lower than Vth, an impulse for lithotrity is not radiated and must wait until the irradiating timing of the subsequent comparison cycle. When such control is performed, lithotrity treatment can be performed without a missed-shot in which a normal tissue other than the stone is irradiated with the impulse. As a result, a side-effect on the patient caused by the lithotrity treatment is decreased.
When the impulse irradiating interval T is excessively short, the patient may sometimes feel pain. Also, since the pulverization pressure can be decreased due to cavitation and the like, the impulse irradiating rate of about 1 to 3 Hz (namely, 1 to 0.33 shot/sec.) is usually employed. When the irradiating rate is, e.g., 1 Hz, the irradiating interval T is 1 second. According to the conventional missed-shot preventive technique described above, the low-pressure wave is sent and received every T second, and coincidence of the focal point (irradiating point) with the stone (i.e., target) is determined from the intensity of the received echo. Therefore, if the focal point and the stone do not instantaneously coincide, radiation must wait for another T second. Impulse lithotrity treatment is generally performed by radiating an impulse onto a target stone several hundreds to several thousands of times or more. Therefore, if the focal point and the stone do not often coincide during treatment, as described above, an accumulated wait time is long and thus cannot be neglected. Time required for lithotrity treatment becomes very long due to the accumulated wait time.
As described above, according to the missed-shot preventive method of the conventional lithotrity apparatus, a low-pressure wave is sent and received between time points corresponding to the impulse irradiating interval, thereby checking whether or not the focal point and the position of the stone coincide with each other. Impulse radiation is controlled based on the determination result of coincidence or non-coincidence. Therefore, if the focal point and the stone do not coincide during a certain comparison cycle, a time until a subsequent cycle is wasted, resulting in a prolonged treatment time for lithotrity.